1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a device and method for transmitting a random access preamble in a wireless communication system.
2. Description of the Related Art
Now, the 3rd Generation Mobile Communication System Partnership Project (3GPP) standardization organization has commenced on Long-term Evolution (LTE) to the existing system criteria. Among many physical layer transmission techniques, both a downlink transmission technique based on Orthogonal Frequency Division Multiplexing (OFDM) and an uplink transmission technique based on Single Carrier Frequency Division Multiple Addressing (SCFDMA) are actively being researched.
In the following description, a sampling frequency of 30.72 MHz is used as an example. In this case, when the interval between sub-carriers is 15 KHz, the number of valid OFDM samples is 2048 and the corresponding sample interval is Ts=1/(15000×2048). For other sampling frequencies, the corresponding number of valid OFDM samples and the number of Cyclic Prefix (CP) samples can be obtained in proportion to the sampling frequency.
There are two types of a frame structure in an LTE system, namely a Frame Structure Type 1 and a Frame Structure Type 2. In Frame Structure Type 1, a Frequency Division Duplex (FDD) mode is employed, and in a Frame Structure Type 2, a Time Division Duplex (TDD) mode is employed. Hereafter, the design of an LTE system implementing the Frame Structure Type 2 with a TDD mode is described.
FIG. 1 illustrates a frame structure of an LTE TDD system according to the related art.
Referring to FIG. 1, a radio frame with a length of 307200×Ts=10 ms for each radio frame is equally divided into two half-frames with a length of 153600×Ts=5 ms. Each half-frame contains eight slots with a length of 15360Ts=0.5 ms and three special domains, i.e., the Downlink Pilot Time Slot (DwPTS), the Guard Period (GP) and the Uplink Pilot Time Slot (UpPTS). The total combined length of the three special domains is 30720Ts=1 ms Each slot contains several OFDM symbols. There are two kinds of CP in OFDM symbols, namely a general CP and an extended CP. A slot with the general CP contains 7 OFDM symbols and a slot with the extended CP contains 6 OFDM symbols. In the application of general CP, the CP in the first OFDM symbol of the slot is 160×Ts (about 5.21 μs) long, and the CPs in the remaining 6 OFDM symbols are 144×Ts (4.69 μs) long. In the application of extended CP, the CP in each OFDM symbols of the slot is 512×Ts(16.67 μs) long. Two continuous slots compose a subframe. Subframe 1 and subframe 6 contain the three special domains. According to the present discussion, subframe 0, subframe 5 and the DwPTS are fixed for downlink transmission. Also, according to the present discussion, for the transition period of 5 ms, the UpPTS, subframe 2 and subframe 7 are fixed for uplink transmission. In addition, according to the present discussion, for the transition period of 10 ms, the UpPTS and subframe 2 are fixed for uplink transmission.
According to the present discussion on LTE TDD, the uplink data, the random access preamble and the channel Sensing Reference Signal (SRS) can be transmitted in the UpPTS.
FIG. 2 illustrates a structure of a random access preamble according to the related art.
Referring to FIG. 2, the random access preamble contains a circular prefix with a length of TCP and a sequence with a length of TSEQ. Several structures of the preamble are defined in the table below:
TABLE 1Parameters for the random access preambleThe preamble formatTCPTSEQ0 3152 × Ts24576 × Ts121012 × Ts24576 × Ts2 6224 × Ts2 × 24576 × Ts321012 × Ts2 × 24576 × Ts4  0 × Ts 4096 × Ts(only for the FrameStructure Type 2)
In Table 1, the preamble format 4 is only applied to an LTE TDD system, the sequence length TSEQ of which is 4096×Ts, which is equal to the time length of two uplink SCFDMA symbols. Here, the CP length TCP, is 0, i.e., no CP is added in the preamble. The feature of such a format is that the random access preamble is short and generally transmitted by virtue of the UpPTS in an LTE TDD system. According to the present discussion, a Random Access CHannel (RACH) signal, which is in this format, is transmitted in the position 5120×Ts prior to the end of UpPTS. Therefore, in the receiving end of a BS, the random access preamble is transmitted within the time segment with a length of 5120×Ts prior to the end of UpPTS. Herein, a random access preamble transmitted through UpPTS is referred to as a short RACH.
According to the present discussion on LTE, the allocation of a frequency band for a Physical Uplink Control CHannel (PUCCH) is implemented at the two ends of the band so as to avoid a Physical Uplink Shared Data CHannel (PUSCH) being divided into multiple frequency bands by the PUCCH. The reason for this is that user equipments transmitting uplink data through multiple frequency bands with no frequency overlap will damage a single-carrier attribute, from which an increase of a Cube Metric (CM) results.
FIG. 3 schematically shows frequency locations of a RACH in an LTE FDD system according to the related art.
Referring to FIG. 3, in each RACH timing location, there are two possible frequencies located at the two ends of the system frequency band, which are adjacent to the PUCCH. This configuration is performed to avoid damaging the PUSCH's single-carrier attribute. According to the present discussion, in an LTE FDD system, only one RACH resource can be configured for one RACH's timing location. RACH's collision probability is controlled by configuring the RACH density in the time domain. In order to obtain the frequency diversity effect, the RACH implements frequency hopping between the two possible frequency locations illustrated in FIG. 3. For an LTE TDD system, it is possible to configure several RACH resources for one RACH timing location to counteract the limitation of an uplink downlink transition period.
It is a typical configuration that the UpPTS contains two SCFDMA symbols. In this case, the UpPTS can be adopted to transmit either the random access preamble in format 4 of Table 1 or the SRS. Suppose at least one RACH resource has been configured in the UpPTS. To guarantee that RACH resources are orthogonal to SRS resources, the SRS can only be transmitted through the rest of the frequency resources. Therefore, a need exists for a RACH configuration method, wherein not only the requirement of the format 4 for the random access preamble can be met, but also the transmission of SRS can be implemented effectively.